Carbon fibers are obtained from carbonization or graphitization of organic fibers and have turbostratic graphitic microstructure. They are inorganic high-molecular-weight fibers having a carbon content above 90%, and are called graphite fibers when the carbon content is higher than 99%. Carbon fibers have high axial strength and modulus, no creep, good fatigue resistance and corrosion resistance, specific heat and conductivity between those of non-metal and metal, a small thermal expansion coefficient, low fiber density and good X-ray transmission, while they are poor in impact resistance, prone to damage, and oxidized under the action of strong acids, and may also lead to phenomena like metal carbonization, carburization and electrochemical corrosion when compounded with metals. Therefore, surface treatment of carbon fibers before use is necessary.
Carbon fibers can be prepared from carbonization of each of polyacrylonitrile fibers, pitch fibers, viscose or phenolic fibers, and can be classified into filaments, staple fibers and chopped fibers according to their state, or into general carbon fibers and high-performance carbon fibers according to their mechanical properties. General carbon fibers have strength of 1,000 MPa and a modulus of approximately 100 GPa. High-performance carbon fibers can be further categorized into the high-strength type (strength: 2,000 MPa; modulus: 250 GPa) and the high-modulus type (modulus: 300 GPa or higher), wherein those having strength higher than 4,000 MPa are called the ultrahigh-strength type and those having a modulus higher than 450 GPa are called the ultrahigh-modulus type. With the advances in the aeronautic and astronautic industry, a high-strength-and-high-elongation type of carbon fibers having elongation greater than 2% has also been developed. Polyacrylonitrile (PAN) carbon fibers are most used and have a market share of 90% or more. Preparation of carbon fibers comprises the following four procedures: fiber spinning (production of precursor), thermal stabilization (pre-oxidation), carbonization, and graphitization, during which the accompanying chemical reactions include dehydrogenation, cyclization, pre-oxidation, oxidation, deoxygenation, and the like.
PAN as the precursor of carbon fibers contains cyano groups (—C≡N), which have a high polarity and impart unique characteristics to the structure and performance of PAN. After sufficient carbonization (at 1,000° C. to 1,500° C.) of the PAN precursor, the mass percentages of N, H and O dramatically decrease and the carbon content reaches 93%-98%, while some doped nitrogen still remains with a nitrogen content of 2%-7%. Graphite fibers are obtained from high-temperature graphitization of carbon fibers at 2,200° C. to 3,000° C., which is a continuation of solid-phase carbonization, driving nearly all non-carbon elements (mainly nitrogen) out of the carbon fibers and leaving graphite fibers having almost 100% carbon. Therefore, one important feature of the PAN-based carbon fibers not treated with the high-temperature graphitization is its distinctive nitrogen-doped structure. For pitch-based carbon fibers, the content of doped nitrogen is low, generally less than 1%, while viscose-based carbon fibers do not contain doped nitrogen.
Thanks to their excellent mechanical property, carbon fibers have been used mainly as reinforcing components in composite materials. Recently, because of their superior conductivity, for example the T700PAN-based carbon fibers produced by Toray Industries, Inc., Japan having resistivity of 1.6×10−3 Ω·cm, carbon fibers have started drawing attentions for their application in electrochemistry, and may be used to produce electrode materials for cells utilizing oxygen dissolved in seawater (metal half-fuel cells using seawater as the medium), proton exchange membrane fuel cells, metal-air fuel cells, microbial fuel cells, a supercapacitor, redox flow cells, lead-acid cells, lithium-ion cells, electrochemical treatment of waste water, an electrochemical transducer, and the like.
Carbon fibers may serve as the electrode material for cathodic oxygen reduction. Oxygen reduction reaction (ORR) plays a vital role in electrochemistry. In various fuel cell technologies, electric powder is generated from an electrochemical reaction constituted by the cathodic oxygen reduction and the anodic oxidation of fuels (such as hydrogen, methanol, active metal, microorganisms, etc.). In the waste water treatment technology by the Electro-Fenton method, cathodic oxygen reduction by electrochemical means produces H2O2 as a continuous source of the Fenton reagent, and the H2O2 reacts with Fe2+ in the solution to produce a highly oxidizing .OH free radical, which can nonselectively destroy nearly all organic contaminants to complete mineralization. Hence, development of carbon-fiber electrode material having excellent electrocatalytic activity for cathodic oxygen reduction offers very bright prospects for various applications.
Fuel cells have been well acknowledged as a clean energy-conversion system. However, their commercialization has been thwarted by two major technical limitations, i.e. cost and reliability. Currently, Pt-based catalysts are the major reason for the high cost of fuel cells, and cheap, highly active and highly stable electrocatalysts for oxygen reduction have been the hot spot of research on fuel cells. In recent years, the fact that nitrogen doping has a significant impact on the performance of carbon and its composite electrocatalyst has attracted wide attention. It has been reported that nitrogen-doped carbon and its composite material have significantly improved catalytic performance which in a basic medium surpasses that of commercial Pt catalysts, and look very promising as a non-noble metal catalyst to replace Pt for use in fuel cells (Gong K, et al., Nitrogen-Doped Carbon Nanotube Arrays with High Electrocatalytic Activity for Oxygen Reduction, Science, 2009. 333: 760-764).
Methods for doping nitrogen in carbon material can be basically classified into (1) in situ doping, wherein nitrogen is doped during synthesis of carbon material; and (2) post doping, wherein synthesized carbon material is subjected to post-treatment with a N-containing precursor (Wen Y, et al., Studies on Nitrogen-Doped Nano-Carbons and Their Non-Pt Metal Composites as Electrocatalysts, Progress in Chemistry, 2010, 22: 1550-1555). In in situ doping, chemical vapor deposition (CVD) is performed on a substrate or template with an organic nitride as a precursor, and, similar to hydrocarbons, the nitride may retain some C—N bonds when the substrate decomposes such that a N-doped nanostructure is formed. In post doping, nano-carbon undergoes post-treatment in a nitrogen-containing atmosphere, so as to afford nitrogen-doped nano-carbon material. Both doping methods are for nano-scale carbon material and generally require a preparation temperature not higher than 1,000° C. A preparation temperature over 1,000° C. may lead to severe escaping of doped nitrogen, which affects the nitrogen doping effect, while an excessively low preparation temperature may also create negative impacts on conductivity of the nitrogen-doped carbon material. In addition, the reactions during the preparation require strict conditions and thus are not suitable for large-scale production, and in actual applications an adhesive agent will be required to manufacture an electrode from the prepared nano-scale nitrogen-doped carbon material.
Commercialized PAN-based carbon fibers are fibrous structures in a size of a few microns, excellent in conductivity, and easy to be processed into an electrode. Commercialized SWB1200 seawater cells (Kongsberg Simrad, Norway), which employ a brush electrode made from PAN-based carbon fibers as the positive electrode for the seawater cells utilizing dissolved oxygen. Although obtained through carbonization at a temperature over 1,000° C., such commercialized PAN-based carbon fibers still have a residual doped nitrogen content of 2%-7%. Therefore, the doped nitrogen contained therein has higher thermal and chemical stability than those obtained by the two doping methods describe above. However, the doped nitrogen inherent in unmodified PAN-based carbon fibers has limited catalytic activity for ORR, and thus has not caught a lot of interest and needs certain modification to obtain good ORR activity (Xu H, et al., Seawater Battery with Electrochemical Supercapacitance, Journal of Electrochemistry, 2012, 18: 24-30).
Previous surface modifications of carbon fibers mainly aim to improve the binding strength between carbon fibers and the composite material, and major modification methods include ozone chemical oxidation and electrochemical anodic oxidation. PAN-based carbon fibers have a smooth surface and show chemical inertness, and these characteristics are unfavorable for them to produce good interface binding with a resin substrate. If surface treatment is applied to PAN-based carbon fibers to introduce active groups onto the surface and increase the surface roughness, then the binding of the carbon fibers can be improved and the mechanical performance of resin-based composite material can be enhanced. Among these methods, anodic oxidation is easy to control, can achieve even oxidation of every fiber, has great operational flexibility, is readily applicable to large-scale treatment, and can increase the interlayer shearing strength in carbon-fiber composite material to about 100 MPa by introducing active functional groups such as oxygen- and nitrogen-containing groups to the surface. However, anodic oxidation as a method for improving mechanical performance requires mild oxidation conditions. Furthermore, if anodic oxidation is used alone for treatment, the introduced oxygen-containing functional groups would be mostly located at the carbon basal planes, and the introduced nitrogen-containing functional groups would be an imino (—NH) or amino (—NH2) group, in which case the introduced doped nitrogen comes from the compounds in the anodic oxidation solution and is not a nitrogen-containing functional group formed from the inherent doped nitrogen previously present in the carbon fibers. In addition, these oxygen- and nitrogen-containing functional groups fail to exhibit effective pseudocapacitive characteristics and electrocatalytic activity for cathodic oxygen reduction, and thus cannot meet the requirements on electrode material.
CN101697323A discloses an electrochemically modified graphite electrode, in which a graphite body is subjected to cyclic treatment between electrochemical oxidation and electrochemical reduction in an aqueous solution of an electrolyte, so as to directly obtain a rough, porous activated layer having certain thickness, abundant oxygen-containing active functional groups and a microcrystalline flake-like structure. The reversible redox reactivity of these oxygen-containing active functional groups may be used for an electrochemical capacitor. CN102176380A discloses a redox reaction electrochemical capacitor, and discloses that the electrochemically modified graphite electrode also has electrocatalytic activity for the redox couples frequently used in redox flow cells. Since graphite itself does not contain oxygen, the surface of the graphite electrode obtained after the above electrochemical treatment does not have a nitrogen-containing active functional group, and thus does not have the characteristics of nitrogen-doped material.
Furthermore, the electrochemical capacitor is characterized by high power, and the fuel cell is characterized by high energy density. Since they are individual devices, a combination thereof is needed to satisfy the requirement for both high power and high energy density with respect to power performance. If they are combined in one unit, the volume of the system will be reduced. This sets out a requirement for electrode material having the characteristics of both the electrochemical capacitor and the fuel cell (mainly depending on the ORR performance).
In summary, development of an oxygen and nitrogen co-doped PAN-based carbon fiber prepared by electrochemical modification remains a crucial problem to be eagerly solved in the field of electrochemistry of material.